1. Field of Invention
The invention relates to a lens and particularly to a lens which exhibits a large depth of focus.
2. Description of the Prior Art
Lenses with two or more simultaneous foci or powers are known. Such lenses, in particular in the form of contact or intra-ocular lenses, are used to correct presbyopia. The drawback of such lenses is that imaging is provided in distinct powers or foci, i.e. a “correct” image produced by the “correct” power is accompanied by a “wrong” image provided simultaneously by a “wrong” power. As a consequence, the lens user frequently experiences ghosting and halos with such lenses.
It is known that a lens of very small diameter exhibits a large depth of focus. This fact can be explained by wave optics considerations and is called the “stenopeic effect”. As an example, a “pinhole lens” of 1.3 mm diameter exhibits a usable depth of focus of about 2.5 diopters. This means that objects can be seen clearly even if they are out of focus by +/−1.25 diopters from the refractive focus of this pinhole lens. Such a pinhole lens is therefore suitable to correct, or, more appropriately, “mask” presbyopia, since such a pinhole lens produces a clear image of objects the distance of which is between infinity (distance vision) and about 40 cm (near vision) in front of this lens, or, if the lens is used as a contact lens, in front of the eye.
It is further known, that such a pinhole lens also masks astigmatism within its available depth of focus. It is actually part of the diagnostic practice in the identification and quantification of the amount of astigmatism present in a human eye.
It should be noted that imaging with a pinhole lens of large depth of focus is absolutely free of ghosting, since imaging is not provided in two or more distinct foci or powers. However, the drawback of such pinhole lenses is reduced throughput of light intensity. As a consequence, imaging of distant and near objects is not satisfactory in dim light conditions.
This drawback can be eliminated with “zoned lenses” formed according to my U.S. Pat. No. 5,982,543 (W. Fiala), the disclosure of which is incorporated by reference herein. It is shown there that an annular lens of small total area exhibits a large depth of focus as well. It is further shown there that an assembly of such annular lenses or annular lens zones provides the same depth of focus like the individual zones, if measures are taken such that the individual contributions of the individual zones add incoherently. An analytical treatment of such lenses can be found in: “W. Fiala et al: Numerical Calculation of a Giant Pinhole Lens in Polychromatic Light”; Annual Report 1996, page 39, editor: Physikalisches Institut, Universitaet Erlangen-Nuernberg”, the disclosure of which is incorporated by reference herein.
In a lens according to U.S. Pat. No. 5,982,543 a rather large number of annular zones is required in order to provide a sizeable depth of focus. In such a lens design, a depth of focus of at least one diopter requires lens zones the maximum area of which is limited to a value F=0.0056 λmm2, F being the maximum area of any of the lens zones, and λ the average wavelength in nanometers. Assuming the common value λ=550 nm, the maximum zone area is calculated to be 3.08 mm2.
In order to achieve a depth of focus of 2.5 diopters with a lens made according to my U.S. Pat. No. 5.892,543, the area of the individual lens zones exhibits a value of approximately 1.33 mm2; this means that a lens of 6 mm diameter would have to comprise twenty-one (21) such zones. Although the production of such a lens is possible, in principle, the production requirements are high.